1. Field of the Invention
This invention relates to a solid state image pickup device represented by a photoelectric conversion element such as a C-MOS, CCD, etc., and also to a method for manufacturing the solid state image pickup device. In particular, this invention relates to a color filter to be configured in conformity with a photoelectric conversion element.
2. Description of the Related Art
A solid state image pickup device, such as a CCD, C-MOS, etc., which is adapted to be mounted in a digital camera has been increasingly enhanced in terms of the number of pixel (picture elements) as well as the fineness of pixels in recent years. In the case of a solid state image pickup device having especially fine pixels, the pixel size is reduced to a level of less than 2 μm×2 μm.
Further, the solid state image pickup device is now configured to have color filters corresponding to photoelectric conversion elements, thereby enabling it to reproduce full color. As for the method of forming the color filter, there has been generally employed a technique of forming a pattern by means of photolithography process (see for example, JP Patent Laid-open Publication (Kokai) No. 11-68076 (1999).
Meanwhile, the region (aperture) which is available to the photoelectric conversion element of solid state image pickup device for the photoelectric transferring is limited to about 20 to 40% based on the entire area of the solid state image pickup device though it depends on the size of the solid state image pickup device and on the number of pixel. Therefore, as the aperture is small in size, it will inevitably result in the deterioration of sensitivity of the solid state image pickup device. In order to make up for this problem, it is generally practiced to provide a condensing micro-lens over the photoelectric conversion element.
However, there has been increasing demand in recent years for a highly refined solid state image pickup device having as many pixels of not less than six millions and hence the size of pixel of color filter to be mounted together with the solid state image pickup device is, in many cases, confined to a level of less than 2 μm×2 μm. This in turn raises a problem that due to insufficient resolution of the color filter to be formed by means of photolithography process, the properties of the solid state image pickup device are badly affected. This insufficiency of resolution is manifested as color uneven originating from the malformation of pattern as the size of pixel becomes as small as not more than 2.5 μm or around 1.8 μm.
Namely, as the size of pixel becomes smaller, the aspect ratio of pattern becomes larger (the thickness of pattern becomes larger relative to the width thereof), so that it is impossible to completely eliminate a portion of color filter that should be essentially eliminated (a portion other than the effective region of pixel), thus permitting it to remain as a residue giving an adverse influence to the pixels of other colors. With a view to overcome this problem, a method has been tried to prolong the developing time. However, this raises another problem that when the developing time is prolonged, a portion of the color filter (pixel) that has been cured and essentially required to remain may be also peeled away.
Further, in the case of the patterning by means of photolithography, there will be raised a phenomenon that the edge portion of pattern of color filter is caused to rise (i.e., a horn-like edge is caused to be generate). Especially when the size of pixel becomes smaller, the properties of the color filter would be adversely affected by this horn-like edge, giving rise to the generation of color uneven.
If it is desired to secure satisfactory spectral characteristics, it would be inevitable to increase the film thickness of the color filter. When the film thickness of the color filter is increased, the edge portion of the pattern of color filter tends to become roundish as the fineness of the pixel is further advance, thus more likely deteriorating the resolution of the color filter. The color filter is generally formed by making use of a photosensitive resin incorporating color pigments. Therefore, when the concentration of pigments included in color filter layers is increased, a quantity of light which is required for the photo-setting reaction of the resin may not reach to the bottom of the color filter layers, thus making it impossible to sufficiently cure the photosensitive resin. As a result, there will be raised a problem that the color filter layers may be peeled off in the developing process of photolithography and hence defective pixels would be caused to be generated.
Furthermore, when the color filter is formed thick, in addition to the aforementioned problems involved in the manufacturing process thereof, there is another problem that the light entering obliquely into a portion of the color filter pattern may be permitted to pass, through a neighboring portion of the color filter pattern, into a photoelectric conversion element, thus raising problems such as mixing of colors and deterioration of sensitivity. This problem becomes more prominent as the size of pixel of the color filter becomes smaller.
In view of the aforementioned phenomena, when it is desired to increase the number of pixel of the solid image pickup element, the problem which is important to deal with is how to make thinner the color filter layer in addition to achieving a highly refined pattern of the color filter.
Incidentally, the problem of color mixing of incident light would be raised even in a case where the distance between the color filter and the photoelectric conversion element is relatively large.
The decrease of aperture ratio of the micro-lens (i.e., decrease of photosensitivity) to be mounted on the highly refined solid state image pickup device and also the deterioration in quality of image due to the increase of noise such as flare and smear are now becoming great issues to be dealt with. Therefore, it has been considered necessary to enhance the converging property of incident light entering into the photoelectric conversion element by making use of micro-lens and to minimize the under-lens distance for enhancing the S/N ratio at the photoelectric conversion element. If the under-lens distance is relatively large, a couple of problems will be raised as follows.
First, if the under-lens distance is relatively large, the uptake angle of incident light becomes smaller, so that the quantity of incident light is decreased, thus providing a dark display as a whole. Secondary, in the case of a camera using a photoelectric conversion element such as a CMOS or CCD, the angle of incident light is generally caused to change depending on the magnitude of diaphragm (F number) of the objective lens. Therefore, when the diaphragm is actuated to move to the full open side, oblique incident light is caused to increase, thus deteriorating the converging property of incident light and hence deteriorating the sensitivity of the photoelectric conversion element. Additionally, since the angle of incident light is caused to differ prominently between a central portion and a peripheral portion of the pixel region of semiconductor chip where a photoelectric conversion element is formed, the quantity of incident light entering into the pixels (photoelectric conversion elements) of the peripheral portion is caused to decrease, thus presenting a dark display at the peripheral portion of the display picture.
The color filter is generally formed on a flattening layer that has been formed in advance on a semiconductor substrate for the purpose of enhancing the adhesion of the color filter to the underlying layer. However, if it is desired to minimize the aforementioned under-lens distance and to miniaturize the solid state image pickup device, it is desirable to dispense with the flattening layer. However, since a color resist to be employed in the photolithography process is poor in adhesion to a semiconductor substrate, it will be peeled away in the developing step. Therefore, it has been considered difficult to dispense with the flattening layer.
With a view to overcoming this problem, there has been proposed a method wherein the surface of semiconductor substrate is treated with chemicals so as to introduce a functional group exhibiting excellent bonding property to a resin into the surface of semiconductor substrate. Even with this method however, it has been impossible to secure a sufficient adhesion of the color filter to the surface of semiconductor substrate.
Meanwhile, the color filter is generally constituted by filters of three primary colors, i.e., blue, green and red filters. There has been a problem however in designing the solid state image pickup device that, due to the characteristics of coloring material, a green resist for forming the green filter is lower in refractive index after the curing thereof as compared with a red resist and a blue resist to be employed for forming the red filter and the blue filter, respectively. Namely, since the color resist to be employed in the photolithography process is required to be excellent in photosensitivity, it is difficult to select one which is also high in refractive index after the curing thereof, thus creating a discrepancy in refractive index among these three different color filters. Because of this discrepancy, the light converging effect by the micro-lens is also caused to differ among these color filters, thus raising a problem that non-uniformity of reflectance is caused to be generated among these color filters.
As described above, the color filter to be created by means of the conventional photolithography process is accompanied with various problems that it is impossible to secure sufficient resolution, that residues of color filter tends to remain unremoved, that the peeling of pixel is more likely to occur, and that the characteristics of the solid state image pickup device may be deteriorated. Additionally, there are also problems that not only the distance between the color filter and the photoelectric conversion element but also the distance between the micro-lens and the photoelectric conversion element (under-lens distance) is caused to become large.